Technological development and increased demand for mobile equipment have lead to a rapid increase in the demand for secondary batteries as an energy source. Among these secondary batteries, a great deal of research and study has been focused on a lithium secondary battery having high energy density and discharge voltage and thus such lithium secondary batteries have been commercialized and entered wide use.
The lithium secondary battery uses a metal oxide such as LiCoO2 as a cathode active material and a carbonaceous material as an anode active material, and is prepared by disposition of a porous polymer separator between the anode and cathode and addition of a non-aqueous electrolyte containing a lithium salt such as LiPF6. Upon charging, lithium ions exit from the cathode active material and migrate to enter into a carbon layer of the anode. In contrast, upon discharging, lithium ions exit from the carbon layer and migrate to enter into the cathode active material. Here, the non-aqueous electrolyte serves as a medium through which lithium ions migrate between the anode and cathode. Such a lithium secondary battery must be basically stable in a range of operating voltage of the battery and must have ability to transfer ions at a sufficiently rapid rate.
When the non-aqueous electrolyte only uses, as a component, a cyclic carbonate having high polarity to sufficiently dissociate lithium ions, this may results in problems associated with increased viscosity of the electrolyte and thus decreased ionic conductivity.
Therefore, U.S. Pat. Nos. 5,521,027 and 5,525,443 disclose a mixed electrolyte of linear carbonates having low polarity but low viscosity to reduce viscosity. Representative examples of linear carbonates may include dimethyl carbonate (DMC), diethyl carbonate (DEC) and ethylmethyl carbonate (EMC). Among them, EMC having the lowest freezing point of −55° C. exerts superior low-temperature performance and life performance when it is used. As examples of cyclic carbonates, mention may be made of ethylene carbonate (EC), propylene carbonate (PC) and butylene carbonate (BC). Inter alia, PC has a low freezing point of −49° C. and thus exerts good low-temperature performance. However, when graphitized carbon having a large volume is used as the anode, PC sharply reacts with the anode during a charging process, and therefore it is difficult to use large amounts of PC. For this reason, EC, which forms a stable protective film at the anode, is primarily used. However, it cannot be said that EC is completely free of reactivity, and therefore decomposition of the electrolyte, which occurs at the anode and cathode during charging/discharging of the battery, is one of numerous causes that shorten a battery life, and particularly EC exhibits increased reactivity at high temperatures, thus resulting in problems.
As an attempt to overcome such problems and thereby improve the battery life at room temperature and high temperature, Japanese Patent Laid-open Publication No. 2000-123867 discloses a battery in which small amounts of ester compounds having a cyclic molecular structure and C═C unsaturated bonds within the ring (for example, vinylene carbonate) were added to the electrolyte. It is believed that such additive compounds decompose at the anode or cathode and then form films on the surfaces of the electrodes, thereby inhibiting decomposition of the electrolyte. However, such additives also cannot completely prevent decomposition of the electrolyte.
In addition, Japanese Patent Laid-open Publication No. 2002-25611 discloses a battery in which ethylene sulfite and vinylene carbonate were added to the electrolyte, and Japanese Patent Laid-open Publication No. 2002-270230 discloses a battery in which various kinds of ethylene sulfite compounds were added to the electrolyte. However, it was also confirmed that those additives disclosed in the above-mentioned prior arts did not exert a desired degree of effects. Further, as battery performance at high temperatures has become gradually important, there is an urgent need for the development of more effective additives.
Therefore, the present invention has been made to solve the above problems and other technical problems that have yet to be resolved.
As a result of a variety of extensive and intensive studies and experiments to solve the problems as described above, the inventors of the present invention have discovered, as will be described hereinafter, the facts that, in a lithium secondary battery comprised of a cathode containing a lithium-containing transition metal oxide, an anode containing graphitized carbon, a porous separator and an electrolyte containing a lithium salt, addition of an ammonium compound capable of providing ammonium ions to the electrolyte results in increased residual capacity and recovery capacity after high-temperature storage of the battery and thereby it is possible to prepare a lithium secondary battery having improved high-temperature performance. The present invention has been completed based on these findings.